The present invention is directed to the preparation of "silacrowns" or "silacrown ethers"--macrocyclic multidentate ethers which resemble in structure and complexation properties a class of compounds known as "crown ethers", but differ in the replacement of a --C.sub.2 H.sub.4 -- group by a silicon group.
Since 1967 when C. Pedersen discovered the crown ethers, literally thousands of applications have developed in which their ability to complex metal ions, solvate inorganic and organic salts in polar and non-polar solvents, and facilitate anionic reactions have been exploited. Much of this work has been reviewed in Synthetic Multidentate Macrocyclic Compounds by R. Izatt and J. Christiansen, Academic Press 1978. Two obstacles have prevented their wider utilization, particularly in commercial processes: current synthetic methods are extremely costly, and the materials have generally high levels of toxicity. These factors, coupled with the difficulty in separating the crown ethers during preparation by processes other than distillation, have hindered wider applications. An example is the acylation step in penicillin synthesis.
Although cyclic polyethyleneoxysilanes have been previously reported, the ring structures have fewer members than the silacrowns. The inside diameters of the ring structures are clearly smaller than lithium ions. R. Kieble, C. Burkhard, J. Am. Chem. Soc. 69, 2689 (1947). Because the ring structures are so small, these compounds cannot form complexes with cations.